This invention relates to a method for preparing a light color resin using a norbornene compound.
A resin of light color is used as a tackifying resin for an adhesive or as a binder for ink or paint. Above all, in the field of a hot melt type adhesive, it is extensively used as a tackifying resin having superior quality.
As the tackifying resin for an adhesive, a petroleum resin, produced upon polymerizing fractions by-produced from the process of naphtha cracking, is generally employed. However, such a resin has a hue of yellow or brown which is markedly distinct from light color.
As one of the methods for preparing the light color resin, there are disclosed in U.S. Pat. No. 1202802 and Japanese Patent Publication No. 17075/1970 a method comprising hydrogenating a petroleum resin or a dicyclopentadiene resin. Since one of the reasons why the petroleum resin is colored resides in the presence of double bonds in the resin, these double bonds are acted upon by hydrogenation to turn the color into light color. However, elevated temperatures and pressures are necessitated in hydrogenating the resin, while the operation of the hydrogenating system is always accompanied by risk. The construction of the system is also costly.
As another method, it is also known to polymerize pure monoolefins to produce a resin of light color substantially free from double bonds. On the other hand, the light color resin of the aromatic series resin is prepared by polymerizing pure aromatic monoolefins, such as styrene or .alpha.-methylstyrene. However, the resin in its entirety is of the aromatic skeleton in this case and hence has a narrow range of compatibility with polymer components of an adhesive so that the usage and application of the resin is restricted.
Turning to a dicyclopentadiene resin, there is disclosed in Japanese Patent Publication No. 11818/1972 a method comprising hydrogenating one of two double bonds of dicyclopentadiene of the starting material in the presence of nickel or palladium catalysts to give dihydrodicyclopentadiene, which is then polymerized to produce the resin, as indicated by the following reaction schema: ##STR3##
The reaction condition in this case is mild since it is not the resin but the monomer that is hydrogenated. In this reaction, the double bond of the norbornene ring is selectively hydrogenated and the double bond of the cyclopentene ring remains in the ultimately produced 9,10 dihydrodicyclopentadiene. However, the double bond of the cyclopentene ring exhibits only poor polymerizability as compared with that of the double bond of the norbornene ring, such that it is difficult to produce the resin with a high yield with the use of 9,10-dihydrodicyclopentadiene as a starting material. On the other hand, a larger amount of the polymerization catalyst need be used for producing the resin with a high yield, in which case, however, the resin is affected in hue significantly.
The norbornene compounds represented by the formula (1), described hereunder, other than dicyclopentadiene, are also not suited as the starting material for preparing the resin having light color since hydrogenation thereof in the known manner results in only the double bond of the norbornene ring being hydrogenated and the other double bonds of lower reactivity remaining unhydrogenated.
While it is thought possible to produce the resin of light color with a high yield by polymerizing a compound having the formula (2) described hereunder, containing only the double bond of the norbornene ring having higher polymerizability, such as 1,2-dihydrodicyclopentadiene represented by the following formula ##STR4## it is practically difficult to produce the resin with a high yield and high purity since the compound of the formula (2) is conventionally synthesized by the Diels-Alder reaction between cyclopentadiene and monoolefin, such that it is technologically impractical and undesirable to produce the compound (2) and to polymerize the compound to produce the resin having light color.